1. Field of the Invention
The present invention relates to a semiconductor device, such as an IGBT (insulated gate bipolar transistor), a MOSFET, a diode, or the like, that has a thin back surface region diffusion layer and a back surface electrode on the back surface side of a semiconductor substrate.
2. Description of the Related Art
IGBTs are voltage driven devices exhibiting a low ON voltage and a high switching speed. IGBTs have been utilized in diverse applications ranging from industrial inverters to household appliances such as microwave ovens. There are several types of structures in IGBTs including a PT (punch through) type, a NPT (non-punch through) type, and an FS (field stop) type. In the following description, “n” represents an n conductivity type and “p” represents a p conductivity type. A PT-IGBT (punch through insulated gate bipolar transistor) is formed using an epitaxial wafer, in which an n buffer layer and an n drift layer are epitaxially grown on a p semiconductor substrate. As a result, a wafer of such a device having a withstand voltage of 600 V has a thickness, for example, which ranges from about 200 to about 300 μm.
FIG. 5 is a sectional view of an essential part of an NPT-IGBT (non-punch through type insulated gate bipolar transistor). FIG. 5 is a sectional view of one cell in an NPT-IGBT chip, the latter including a multiple of cells.
As shown in FIG. 5, a p base region 3 is selectively formed in the front surface side of an n drift layer 2, which is an n semiconductor substrate 1 composed of an FZ wafer, for example, and not a diffusion layer. An n emitter region 4 is selectively formed in the front surface side of the p base region 3. A gate electrode 6 is formed on the front surface of the substrate and stretches from one n emitter region 4 in one p base region 3 to another n emitter region 4 in another p base region 3 which is separate from the former p base region 3, as shown in FIG. 5, with interposing of a gate insulator film 5 under the gate electrode 6.
Emitter electrode 8 is in contact with both the n emitter regions 4 and the p base regions 3, and insulated from the gate electrode 6 by an interlayer insulator film 7. On the back surface of the substrate 1, a p collector layer 10 and a back surface electrode 54 are formed in which the latter is a collector electrode. The p collector layer 10 and the back surface electrode 54 make up a back surface region structure 55. The reference numeral 18 in FIG. 5 designates a front surface region structure, the numeral 21, a solder, the numeral 22, a support conductor, and the numeral 60, a chip after cutting the wafer. The thickness of the n drift layer 2 of an NPT-type IGBT is greater than that of a PT-type IGBT. On the other hand, the p collector layer 10 in the NPT-type IGBT, when formed by ion implantation from the back surface side, can be made significantly thinner than the p collector layer 10 in a PT-type IGBT, which employs a high density p semiconductor substrate for a p collector layer. Therefore, wafer thickness can be remarkably reduced as compared with a PT-type IGBT device.
Recently, in order to reduce the ON voltage and the switching loss, FS-IGBTs (field stop type insulated gate bipolar transistors) have been developed that have an n semiconductor substrate with a reduced thickness and a back surface region diffusion layer (a field stop layer and a p collector layer) with a reduced thickness.
FIG. 6 is a sectional view of an essential part of an FS-IGBT. FIG. 6 is a sectional view of one cell in an FS-IGBT chip, the latter including a multiple of cells therein. As shown in FIG. 6, the device structure in the front substrate surface side (a front surface region structure 18) is the same as the front surface region structure 18 in the NPT-type device shown in FIG. 5. In the back surface region of the substrate, a buffer layer 9 (which is called a “field stop layer 9” in an FS-IGBT) is provided between the n drift layer 2 and the p collector layer 10. In the FS-type device, since the n semiconductor substrate 1 can be made very thin, the wafer thickness is remarkably reduced as compared with a PT-type device. Furthermore, the thickness of a wafer in the FS-type device, which includes a field stop layer 9, can be reduced as compared to that of a NPT-type device.
In this FS-IGBT, the thickness of the n semiconductor substrate 1 ranges from approximately 80 μm to 100 μm in a class of devices having a withstand voltage of 600 V, from approximately 100 μm to 140 μm in a class of devices having a withstand voltage of 1,200 V. The thickness of the filed stop layer 9, which stops the spread of a depletion layer, is about 1 μm. A p collector layer 10 with a thickness of about 1 μm is formed in contact with this field stop layer 9, forming a back surface region diffusion layer 16. On the surface of this collector layer 10, a back surface electrode 54, which becomes a collector electrode, is formed. The back surface electrode 54 is formed by laminating a plurality of metal films 51, 52.
The back surface electrode 54 consists of a titanium film 51 and a nickel film 52, in the order from the side in contact with the p collector layer 10, and joined to a support conductor 22 (such as a metal base) with a solder 21. Reference numeral 55 in FIG. 6 designates a back surface region structure, and reference numeral 60 designates a chip after cutting.
The front surface region structure 18 in FIG. 5 and FIG. 6 consists of p base region 3, n emitter region 4, gate insulator film 5, gate electrode 6, interlayer insulator film 7, emitter electrode 8, and a protective film (not shown in the figures). The back surface region structure 55 consists of the back surface region diffusion layer 16 and the back surface electrode 54.
FIGS. 7(a) through 7(c) illustrate a method of manufacturing the FS-IGBT of FIG. 6. FIGS. 7(a) through 7(c) are sectional views showing steps to manufacture essential parts in the order of the manufacturing steps. The reference numeral 18 designates the front surfaced region structure 18 and the reference numeral 55 designates the back surface region structure, details of the structures thereof being omitted in the figures for simplicity and clarity.
Referring to FIG. 7(a), a front surface region structure 18 is formed in the first principal surface region of wafer 30a. The back surface 20a is ground away to thin the wafer 30a down to a thickness of 140 μm. The reference numeral 30 in FIG. 7(a) designates the wafer that has been thinned by grinding which is being worked into n semiconductor substrate 1.
Referring to FIG. 7(b), a titanium film 51, a nickel film 52 and a gold film are laminated on the p collector layer 10 (which are not shown individually in FIG. 7(b), but are shown as back surface region structure 55) on the back surface 20 of the wafer 30. A region surrounded by scribe lines 62 of the wafer 61, on which the front surface region structure 18 and the back surface region structure 55 are formed, becomes a chip 60 in region 60a for forming the chip 60.
Referring to FIG. 7(c), after forming a chip 60 having a chip size 60 as exemplified, by cutting the wafer 61 along the scribe lines 62, the gold film on the back surface of the chip 60 is joined to a support conductor 22 (a copper base, for example) with a solder 21. This gold film, after joining, is absorbed in the solder 21 and disappears.
The FS-IGBT of FIG. 6 formed in the process as described above, is subjected to measurements of gate characteristics and withstand voltage characteristics during the stage of wafer 61 shown in FIG. 7(b). The measurements made in this stage are intended to find defective chips in this early stage and transfer only good chips to for further processing steps to reduce manufacturing costs.
FIG. 8 shows an arrangement for measuring the withstand voltage characteristics in the wafer stage. The wafer 61 is positioned on a stage 35 and held by pressing the periphery of the wafer 61 with metal fittings 63. A probe 64 is pushed against the surface of the wafer 61. A voltage is applied between collector and emitter of the region 60a for forming a chip 60 in which an FS-IGBT is formed, to measure leakage current using a curve tracer (plotter) 65. The applied voltage is set at a voltage to make a depletion layer reach the n field stop layer 9.
During the measurement, if any dust 36 is present on stage 35 shown in FIG. 8, the wafer 61 is placed on the dust 36 and, when the wafer is contacted and pushed by probe 64, the wafer 61 warps. When the dust has a particle size which is large enough to badly warp the wafer 61, a crack may result and the chip 60a may be rendered defective. When the dust 36 has a smaller particle size, the n field stop layer 9 in the area placed on the dust 36 distorts from the pressure exerted by the metal fitting 63 generating strain shown generally in area “O”. Then, when the depletion layer reaches the n field stop layer 9, the leakage current increases abruptly due to the piezoelectric effect, and the chip is falsely judged to be defective. In this case, however, if the measurement is repeated after removing dust 36 to thus eliminate the strain, the chip tests as a good chip. A major type of dust 36 encountered consists of fragments of silicon particles broken from the periphery of the wafer during the manufacturing process. In the case a thick n field stop layer 9, however, an increase of leakage current due to the piezoelectric effect does not occur.
FIG. 9 shows the correlation between the rate of false judgment of leakage current and the chip size. The leakage current is measured for each chip 60a in the state of a wafer 61 as shown in FIG. 8. The chip size is the size of the region 60a for forming a chip as indicated in FIG. 7b. As can be seen from FIG. 9, the rate of false judgment rapidly increases above a chip size of 8 mm square, and becomes more than 60% at 11 mm square.
A rate of false judgment in leakage current measurement of chips is defined by the expression:(A−B)/A)×100(%)wherein A is the number of defective chips determined by leakage current in the measurement on a wafer 61, and B is the number of defective chips determined by leakage current measured on the chips cut out from the places of the chips that have been judged as defective. Initially, an address number is given to every region 60a for forming a chip in the stage of a wafer 61 shown in FIG. 7b. Leakage current is measured at every region 60a for forming a chip in the state of a wafer 61 to obtain the number of defective chips A. After dividing into chips, the leakage current is measured on the chips with the address number judged as defective, to obtain the number of defective chips B. By giving the address number to the regions 60a for forming a chip, it becomes possible to obtain the number of chips that have turned out to be good after being divided into chips from the regions 60a for forming a chip which were initially classified to be defective in the stage of a wafer 61. Here, such chips are excluded in the count of chips that have turned out defective in the leakage current measurement due to clacking or breaking during the process of dividing into chips. In the leakage. current measurement performed on each chip 60, the stage 35 is placed in a dust-free state by thorough cleaning prior to contact with the chip 60. Since the stage 35 on which a chip 60 is placed has a size which is approximately the same as that of the chip 60, and is smaller than one part in several tens of the size of the stage 35 on which a wafer 61 is placed, dust 36 can be thoroughly eliminated.
Japanese Unexamined Patent Application Publication No. 2004-103919 discloses a semiconductor wafer having semiconductor devices formed in the first principal surface region (front side) of the wafer and an electrode film on the second principal surface (back side) of the semiconductor wafer, in which a metal layer is formed on the second principal surface side interposing a titanium layer there between. This structure is stated to give a semiconductor wafer that is thin as finished and hardly warped.
Japanese Unexamined Patent Application Publication No. 2003-282589 discloses a process for making a wafer in which an impurity diffusion region for forming a semiconductor device in the surface region of one side of the wafer is provided, which is ground down to a predetermined thickness from the other side of the wafer; the wafer is etched to thin down to a predetermined thickness excepting the peripheral region; an impurity-doped polysilicon film is formed on this etched surface, from which impurities are diffused to form an impurity diffusion region for a contact; and a back surface electrode is formed in contact with the polysilicon film. This document asserts that this structure avoids the strength issue typical in a thin wafer and attains a contact on the back surface electrode at a relatively low temperature. This back surface electrode consists of a titanium film, a nickel film, and a gold film in this sequence from the polysilicon film in the state of a wafer.
Japanese Unexamined Patent Application Publication No. 2001-135814 discloses a vertical type MOSFET having a Schottky junction on the back surface side, in which the Schottky junction is formed using an Al—Si alloy with a thickness of 1,500 Å and a silicon content of at least 0.5 wt %. This document asserts that this structure achieves lower losses and costs.
Japanese Unexamined Patent Application Publication No. 2005-244165 discloses a wafer prepared for making semiconductor chips in which the principal front surface and the principal back surface of the wafer are in conformity with the principal front surface and the principal back surface of the semiconductor chips, respectively, and a back surface electrode is formed on the principal back surface of the semiconductor wafer. In the condition when the back surface electrode is fixed on a support conductor, after forming a front surface electrode on the principal front surface of the semiconductor wafer, the support conductor is removed and the semiconductor wafer is cut to form semiconductor chips. This document asserts that the distortion of the semiconductor wafer for making semiconductor chips is suppressed and is minimal. The back surface electrode consists of three layers, i.e., a titanium film, a nickel film, and a gold film in this sequence from the semiconductor side.
As described above with reference to FIGS. 6 through 9, when the back surface region diffusion layer 16 is thin and dust 36 is present on the stage 35 for measuring the characteristics, the region 60a for forming a chip in the wafer 61 suffers from cracking or from false judgment of leakage current thereby lowering the proportion of good chips deemed obtained. Moreover, when the particle size of dust 36 is about 10 μm or less, the proportion of false judgment of leakage current due to piezoelectric effect increases in the region 60a for forming a chip.
The four Japanese Unexamined Patent Applications discussed in the foregoing, however, do not mention the false judgment of leakage current at the region for forming a chip in the stage of a wafer, which is a problem when the back surface region diffusion layer is thin.
It is therefore an object of the present invention to solve the above problem and provide a semiconductor device in which the false judgment of leakage current due to piezoelectric effect at the region for forming a chip in the state of a wafer scarcely occurs even when dust is present on the stage for measuring the device characteristics.